1. Field of the Invention
The invention relates to a speed reduction mechanism that is suitably used in, for example, an electric vehicle that has an electric motor as a driving source, and a motor torque transmission device that includes the speed reduction mechanism.
2. Description of Related Art
There is a conventional motor torque transmission device that is mounted on a vehicle, and that includes an electric motor and a reduction-transmission mechanism (see, for example, Japanese Patent Application Publication No. 2007-218407 (JP 2007-218407 A)). The electric motor generates motor torque. The reduction-transmission mechanism reduces the speed of rotation transmitted from the electric motor and then transmits driving force to a differential mechanism.
The reduction-transmission mechanism of the motor torque transmission device of the above-described type has a pair of disc-shaped revolving members, a plurality of outer pins and a plurality of inner pins. The revolving members make revolving motions in accordance with the rotation of the motor shaft of the electric motor. The motor shaft has eccentric portions. The outer pins apply rotation force to the revolving members. The inner pins are arranged radially inward of the outer pins, and output the rotation force of the revolving members to the differential mechanism as torque.
The revolving members each have a center hole and a plurality of pin insertion holes. Each center hole extends along the central axis direction of the corresponding revolving member and opens toward both sides of the revolving member in the central axis direction. The pin insertion holes are arranged around the central axis of the corresponding center hole at equal intervals. The revolving members are rotatably supported by the eccentric portions of the motor shaft via bearings (cam-side bearings).
The outer pins are arranged at equal intervals around the axis of the motor shaft with the eccentric portions, and are fitted to a housing for the reduction-transmission mechanism.
The inner pins are passed through the pin insertion holes of the revolving members. The inner pins are arranged at equal intervals around the axis of the motor shaft with the eccentric portions, and are fitted to the differential case. Bearings (pin-side bearings) are fitted to the inner pins. The bearings are used to reduce contact resistance between the inner pins and the inner peripheries which define the pin insertion holes of the revolving members.
In the motor torque transmission device described in JP 2007-218407 A, a plurality of the outer pins need to be prepared, and, in addition, the outer peripheral portions of the revolving members need to be formed into a complex shape. Therefore, employing the motor torque transmission device described in JP 2007-218407 A is not economical.
The above-described non-economical factor may be eliminated by forming a reduction-transmission mechanism as follows. That is, revolving members are formed of external gears, a rotation force applying member used to apply rotation force to the revolving members is formed of an internal gear, and the number of teeth of the internal gear is set larger than the number of teeth of each of the external gears.
However, if the above-described reduction-transmission mechanism formed of the external gears and the internal gear is used in a motor torque transmission device for a vehicle, the revolving speed of each of the external gears that are the revolving members becomes relatively high. Therefore, a load due to centrifugal force acts on the pin-side bearings from the revolving members when the torque. Accordingly, bearings having high durability need to be used as the pin-side bearings, resulting in cost increase. In addition, if a load due to centrifugal force acts on the pin-side bearings, the service life of each pin-side bearing decreases.